Although various attempts have been made to form a mouth piece of the precombustion chamber by ceramics mainly for enhancing of the combustion efficiency of diesel engines, thermal insulation only at the mouth piece is not sufficient from the view point of performance. Research has recently been done to form the chamber by ceramics for thermally insulating the same. If the thermal insulator of ceramics is in contact with a hollow space provided in the cylinder head, heat dissipation would be high, resulting in a low thermal insulation effect. Therefore it has been proposed that an air thermal insulation layer be provided around the outer periphery of a ceramic body to compensate for the low thermal insulation. (Refer to Japanese Utility Model Kokai Publication No. B59-21024 entitled "Precombustion chamber for internal combustion engine".)